OBJECTIVE: For Chilean teenage mothers under 15 years old and from 15 to 19 years old, to evaluate the trends in birth rates and reproductive risk for the period of 1990­1999. METHODS: A database was constructed using data from the Demography Yearbook (Anuario de demografía) volumes published by Chile's National Institute of Statistics (Instituto Nacional de Estadísticas) for 1990­1999. From that database we calculated the trends in the number of live births and in the rates of maternal mortality, late fetal mortality, neonatal mortality, and infant mortality among the teenage mothers under 15 and from 15 to 19 years old. We calculated the risk odds ratio (OR) for both of those age groups in comparison with women from 20 to 34 years old. The groups were compared using Fisher's exact test or the chi-square test, and the analysis of trends in the period studied was carried out with Pearson's correlation, with an alpha level of 0.05. RESULTS: In the period studied, for the teenage mothers under age 15, the respective rates for maternal mortality, late fetal mortality, neonatal mortality, and infant mortality were 41.9 per 100 000 live births, 5.1 per 1 000 live births, 15.2 per 1 000 live births, and 27.4 per 1 000 live births. For the adolescents from 15 to 19 years, the corresponding rates were 19.3, 4.1, 8.1, and 16.6; for the women 20­34 years old, they were 26.8, 5.0, 6.7, and 12.1. The adolescents under 15 had higher risks of maternal mortality (OR = 1.56; 95% confidence interval (CI): 0.50 to 4.31; P = 0.372) and of fetal mortality (OR = 1.02; 95% CI: 0.76 to 1.36; P = 0.890), but those differences were not statistically significant. However, the younger adolescents did have significantly higher risks of neonatal mortality (OR = 2.27; 95% CI: 1.92­2.68; P < 0.0001) and of infant mortality (OR = 2.39; 95% CI: 2.04 to 2.62; P < 0.0001). In comparison to the women 20­34 years old, the teenage mothers from 15 to 19 years old had significantly lower risks of maternal mortality (OR = 0.72; 95% CI: 0.56 to 0.92; P < 0.008) and of fetal mortality (OR = 0.81; 95% CI: 0.77 to 0.86; P < 0.0001) but significantly higher risks of neonatal mortality (OR = 1.20; 95% CI: 1.16 to 1.25; P < 0.0001) and of infant mortality (OR = 1.38; 95% CI: 1.35 to 1.42; P < 0.0001). Among both the older teenage mothers and the mothers 20­34 years old there was a significant downward trend in maternal, fetal, neonatal, and infant mortality rates in the period studied; in the younger adolescents only neonatal mortality and infant mortality declined significantly. There was a rising trend in the number of live births among the two groups of teenage mothers, but that trend was statistically significant only for the mothers under 15; among mothers 20­34 years old there was a statistically significant downward trend. CONCLUSIONS: In the period studied, the Chilean teenage mothers faced greater reproductive risk than did the women 20­34 years old. The number of live births among teenage mothers tended to rise during the 1990­1999 period, but the change was significant only for the mothers under age 15. These results point to the need to develop programs that improve both sex education and birth control practices starting in early adolescence.